This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
Wireless communication systems implement a network of base stations or other access nodes to provide wireless connectivity to different geographical areas or cells. User equipment or other access terminals located within the cells can access the wireless communication system by establishing an over the air communication session with one or more access nodes. User equipment may also be able to move or roam between different cells and so the wireless communication system typically implements mobility functionality that allows the system to hand off user equipment between different access nodes and to keep track of the serving access node for the user equipment. However, in some circumstances the network may need or want to determine a more precise geographical location of user equipment. For example, the network may provide location-dependent services that use the geographic location (e.g., coordinates such as latitude-longitude) of user equipment to configure the services provided to the user equipment. For another example, the network may provide emergency services that allow user equipment to be located in emergency, e.g., in response to a 911 call from the user equipment.
Wireless communication devices, particularly mobile user equipment, often implement global positioning system (GPS) functionality to determine the geographic location of the device using multiple GPS satellite signals. However, GPS functionality consumes significant battery power and so users often disable or turn off the GPS functionality when they are not using a specific location-dependent service that requires geographic information. Disabled GPS functionality may be unable to provide requested location information in emergency situations and enabling a disabled GPS function and then acquiring the necessary satellite signals may introduce significant time delays. Furthermore, GPS functionality may not work when the user equipment is obscured or shielded and unable to acquire the necessary number of satellite signals.
The geographic locations of user equipment can also be determined by triangulation or trilateration using distances between the user equipment and multiple base stations. For example, user equipment can determine its geographic location using downlink observed time difference of arrival (OTDOA) measurements for signals transmitted by a group of neighboring base stations. The difference between the arrival times of signals from two different base stations can be used to determine a locus of possible user equipment locations. The locus typically includes more than one candidate location and the redundancy/degeneracy can be broken using one or more additional loci determined using the OTDOA for signals received from another pair of base stations. However, the base stations that transmit the positioning reference signals (PRS) may need to limit or mute transmission concurrently with transmitting the OTDOA signals to reduce interference so that the user equipment can detect the downlink signals with a sufficiently high signal-to-noise ratio (SNR) or signal to interference-plus-noise ratio (SINR). This can significantly impact overall system capacity. Moreover, OTDOA techniques require additional functionality and computing power at the user equipment.